Как записать элемент суммы в цикле? - VB

Private Sub Command1_Click()
Dim x As Single, eps As Single
Dim Sr  As Single, Sk As Single, S As Single, n As Long
x = Val(Replace(InputBox("|x|<1", , 0.2), ",", "."))
eps = Val(Replace(InputBox("eps", , 0.001), ",", "."))
If Abs(x) < 1 And eps > 0 Then
List1.Clear
Sr = 1: Sk = 0: S = 0
n = 1
 Do While Abs(Sr - Sk) > eps
 Sk = Sr
 S = S + f(x, n)
 Sr = 1 - S / 6
 List1.AddItem n & "    " & Round(S, 6)
 n = n + 1
 If n > 400 Then
 MsgBox "слишком высокая точность"
 Exit Sub
End If
 Loop
 
 List1.AddItem "----------"
   List1.AddItem " результат         " & Round(S, 6)
      List1.AddItem "----------"
        List1.AddItem "прямой расчет  " & Round(1 / ((x + 4) ^ 4), 6)
Else
MsgBox "ложный ввод"
End If
End Sub
Function f(x As Single, k As Long) As Single
    ' тут задай правильно
f = ((x * (k + 4)) / ((-1) ^ (k + 1))) '(((-1) ^ (k + 1)) * (x ^ n)) / ((k + 1) * (k + 2) * (k + 3))
 
End Function